Antennas for handheld devices (e.g., smartphones) are relatively complex structures. Modern antenna designs are limited by physical and functional constraints due to the small sizes of handheld devices and functional restrictions imposed by carriers and regulatory agencies. Moreover, a handheld device typically must accommodate numerous antennas, such as a primary cellular antenna, a diversity cellular antenna, a global positioning system (GPS) antenna, a Wi-Fi antenna, a near field communication (NFC) antenna, and the like.
For example, the primary antenna of a smartphone is typically the only cellular antenna that transmits signals. The primary antenna is designed to support specific frequencies, and comply with a limited specific absorption rate (SAR) of energy that can be absorbed by human tissue and a total radiated power (TRP) for every frequency band that the handheld device supports. These constraints, along with the type of antenna, and number of other antennas, typically dictate the location of an antenna on a handheld device. For example, the location of a primary antenna is usually at the lower end of a handheld device to comply with SAR limitations.
Dipole antennas are commonly used in smartphones. A dipole antenna has two conductive elements, such as metal wires or rods, that are usually bilaterally symmetrical. The dipole antenna is electrically coupled to communications circuitry such as transmitter and/or receiver circuitry. In operation, a driving current from the transmitter is applied or, for receiving antennas, an output signal to the receiver is taken, between the two conductive elements of the antenna.
A dipole antenna is physically about a half-wavelength long to provide reasonable efficiency and bandwidth. The overall size of the antenna is determined by the lowest frequency of operation because it has the longest wavelength. For example, supporting a low-band of around 810 MHz requires a handheld device to be about 7 inches long. As a result, an antenna may use an entire structure of a mobile phone, which is about 5 to 7 inches long.
FIG. 1 is a schematic diagram that shows the evolution of a simple dipole antenna into a typical dipole antenna for cellular phones. FIG. 1(a) shows a six-inch center fed dipole that includes two bilaterally symmetrical conductive elements 10-1 and 10-2. FIG. 1(b) shows a non-center fed dipole antenna with one fat arm 10-4. The fat arm 10-4 could make up the chassis for a mobile phone and function as a ground plane of the antenna to serve as a reflecting surface for radio waves. In FIG. 1(c), a top arm 10-5 is meandered to increase the length of the dipole antenna, and from there the antenna can evolve into an inverted-F antenna that is commonly used in wireless communications.
FIG. 2A shows an antenna formed by an encasing of a handheld device 12. As shown, the encasing is formed of three conductive elements 14-1, 14-2, and 14-3 separated by gaps 16-1 and 16-2 including non-conductive material. Examples of conductive material include aluminum and titanium. Examples of non-conductive material include various ceramics. FIG. 2B is a functional representation of the antenna 18 formed by the encasing of handheld device 12. The antenna 18 includes two antenna elements 20-1 and 20-2 corresponding to the physical conductive elements 14-1 and 14-2, respectively.
The gaps 16-1 and 16-2 that physically separate the conductive elements 14-1, 14-2, and 14-3 are commonly referred to as “antenna breaks.” The separation formed by gap 16-1 enables the antenna 18 of handheld device 12 to radiate. This antenna design is difficult to implement because having that much metal on the backside of the handheld device 12 introduces parasitic capacitance that does not radiate. Moreover, the antenna breaks 16-1 and 16-2 are aesthetically unpleasing. Thus, current antenna designs for handheld devices have presented several challenges and are limited as a result of functional and physical constraints.